Methods and apparatuses for managing effluent products in a fuel cell system

ABSTRACT

A water management system for a fuel cell having an anode chamber including a fuel, a cathode chamber in fluid communication with an oxidizing agent, and a proton conducting membrane electrolyte separating the chambers. The system includes a gas plenum, a first valve for controlling a first flow of a gas from the anode chamber into the gas plenum, and a second valve for controlling a second flow of the gas collected by the gas plenum into the cathode chamber. The first valve is opened allowing the first flow while the second valve is closed between the gas plenum and the cathode chamber so that effluent gas is collected in the gas plenum. When the amount of the effluent gas in the gas plenum reaches a predetermined value, the first valve is closed and the second valve is opened to allow the second flow.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates generally to fuel cell systems, and moreparticularly, the invention relates to methods and apparatuses formanagement of effluent products produced during an electrochemicalreaction in a direct oxidation fuel cell system.

[0003] 2. Background of the Invention

[0004] Fuel cells are devices in which an electrochemical reaction isused to generate electricity. A variety of materials may be suitable foruse as a fuel depending upon the materials chosen for the components ofthe cell and the intended application for which the fuel cell willprovide electric power.

[0005] Fuel cell systems may be divided into “reformer based” systems(which make up the majority of currently available fuel cells), in whichfuel is processed to improve fuel cell system performance before it isintroduced into the fuel cell, and “direct oxidation” systems in whichthe fuel is fed directly into the fuel cell without internal processing.

[0006] Because of their ability to provide sustained electrical energy,fuel cells have increasingly been considered as a power source forsmaller devices including consumer electronics such as portablecomputers and mobile phones. Accordingly, designs for both reformerbased and direct oxidation fuel cells have been investigated for use inportable electronic devices. Reformer based systems are not generallyconsidered a viable power source for small devices due to size andtechnical complexity of present fuel reformers.

[0007] Thus, significant research has focused on designing directoxidation fuel cell systems for small applications, and in particular,direct systems using carbonaceous fuels including methanol, butanol,propanol, and formaldehyde. One example of a direct oxidation fuel cellsystem is a direct methanol fuel cell system. A direct methanol fuelcell power system is advantageous for providing power for smallerapplications since methanol has a high energy density (providing compactenergy storage), can be stored and handled with relative ease, andbecause the reactions necessary to generate electricity occur underambient conditions.

[0008] DMFC power systems are also particularly advantageous since theyare environmentally friendly. The chemical reaction in a DMFC powersystem yields only carbon dioxide and water as by products (in additionto the electricity produced). Moreover, a constant supply of methanoland oxygen (preferably from ambient air) can continuously generateelectrical energy to maintain a continuous, specific power output. Thus,portable computers, mobile phones and other portable devices can bepowered for extended periods of time while substantially reducing andpotentially eliminating at least some of the environmental hazards andcosts associated with recycling and disposal of alkaline, Ni—MH andLi—Ion batteries.

[0009] The electrochemical reaction in a DMFC power system is aconversion of methanol and water to CO₂ and water. More specifically, ina DMFC, methanol in an aqueous solution is introduced to an anodechamber side of a protonically-conductive, electronically non-conductivemembrane in the presence of a catalyst. When the fuel contacts thecatalyst, hydrogen atoms from the fuel are separated from the othercomponents of the fuel molecule. Upon closing of a circuit connecting aflow field plate of the anode chamber to a flow field plate of thecathode chamber through an external electrical load, the protons andelectrons from the hydrogen atoms are separated, resulting in theprotons passing through the membrane electrolyte and the electronstraveling through an external load. The protons and electrons thencombine in the cathode chamber with oxygen producing water. Within theanode chamber, the carbon component of the fuel is converted bycombination with water into CO₂, generating additional protons andelectrons.

[0010] The specific electrochemical processes in a DMFC are: AnodeReaction: CH₃OH + H₂O = CO₂ + 6H⁺ + 6e Cathode Reaction: O₂ + 6H⁺ + 4e =2H₂O Net Reaction: CH₃OH + 3/2O₂ = CO₂ + H₂O

[0011] The methanol in a DMFC is preferably used in an aqueous solutionto reduce the effect of “methanol crossover”. Methanol crossover is aphenomenon whereby methanol molecules pass from the anode side of themembrane electrolyte, through the membrane electrolyte, to the cathodeside without generating electricity. Heat is also generated when the“crossed over” methanol is oxidized in the cathode chamber. Methanolcrossover occurs because present membrane electrolytes are permeable (tosome degree) to methanol and water.

[0012] One of the problems with using DMFC power systems in portablepower applications is the lack of a low-cost, effective method andsystem for removing effluents produced by the electrochemical reactiongenerally, and in particular, to remove water generated on the cathodicface of the membrane electrolyte or otherwise present in the cathodechamber. If water generated in the cathode chamber collects on thecathode of the membrane or in the anode chamber, it may prevent oxygenfrom coming into contact with the cathodic electrocatalyst, interruptingproductive oxidation of the fuel and generation of electricity.

[0013] In addition, the proper ratio of fuel to water delivered to theanode chamber in DMFC power systems must be maintained. Duringoperation, water molecules may be pulled across the membrane withhydrogen protons leading to excess water on the cathode side of themembrane and an increase in methanol concentration at the anode. Theincreased concentration of methanol may lead to additional methanolcrossover resulting in decreased efficiency, a waste of methanol, andthe generation of unwanted heat.

[0014] Theoretically, the effluents could be removed by venting thecarbon dioxide out of the anode chamber and evaporating the water fromthe cathode side of the membrane electrolyte with a low humidity ambientairflow. However, under many relevant conditions (e.g., low volume airflow, low ambient air pressure, moderate to high humidity), the watercannot be effectively removed, and thus, alternate methods ofeliminating water generated in the cathode are required.

[0015] According, the suitability of DMFC power systems for poweringportable devices and consumer electronics is dependent upon thedevelopment of systems and methods for eliminating and/or recirculatingthe effluent products produced during operation of the fuel cell. Inaddition, in order for DMFC power systems to be used effectively, theymust be self-regulating and passively generate electrical power underbenign operating conditions, such as ambient air temperature andpressure.

SUMMARY OF THE INVENTION

[0016] Accordingly, the present invention provides a water managementsystem and method for managing effluent products generated as a resultof fuel oxidation in a fuel cell system. More particularly, the presentinvention provides a water management system and method using aneffluent gas (carbon dioxide) generated as a by-product of said fueloxidation to remove or recirculate water from the fuel cell system.

[0017] The water management system and method according to the presentinvention is particularly well suited for use with a direct oxidationfuel cell system. Carbon dioxide produced from the oxidation of fuel isnot directly exhausted from the fuel cell system but instead, used toremove/recirculate effluent water.

[0018] The present invention also provides a system and method forrecirculating effluent water in a fuel cell system to maintain apreferred concentration of the carbonaceous fuel, thereby reducing theamount of water that must be stored with the carbonaceous fuel tomaintain an optimum fuel concentration.

[0019] Accordingly, the below recited aspects of the present inventionare directed to direct oxidation fuel cell systems, and more preferablyto direct methanol fuel cell power systems.

[0020] In one aspect of the present invention, an effluent gas producedin an anode chamber of a fuel cell is collected and then exhaustedthrough a cathode chamber of the fuel cell when the amount of effluentgas reaches predetermined value.

[0021] In another aspect of the present invention, a fuel cell includesan anode chamber having a fuel, a cathode chamber in fluid communicationwith an oxidizing agent, a proton conducting membrane electrolyteseparating the chambers, and a first valve for controlling a first flowof a gas from the anode chamber into the cathode chamber. A relatedmethod for reducing the amount of water in the cathode chamber includesclosing the first valve allowing an effluent gas produced in the anodechamber to collect and opening the first valve when an amount of theeffluent gas reaches a predetermined value.

[0022] In yet another aspect of the present invention, the fuel cellaccording to the second aspect further includes a gas plenum and asecond valve. The first valve controls the first flow of the gas fromthe anode chamber into the gas plenum and the second valve controls asecond flow of the gas collected in the gas plenum into the cathodechamber. A further related method includes opening the first valveallowing said first flow while said second valve is closed between saidgas plenum and said cathode chamber. Effluent gas is then collected inthe gas plenum via the first flow. When an amount of effluent gascollected in the gas plenum reaches a predetermined value, the firstvalve is closed and the second valve is opened, allowing the secondflow.

[0023] In yet another aspect of the present invention, which may be usedin conjunction with the above aspects, a fuel cell includes a fluidplenum, a third valve for controlling the second flow out of an outletof the cathode chamber and into the fluid plenum and out an exhaust portand a fourth valve for controlling a third flow from the fluid plenuminto the anode chamber.

[0024] The third valve of the fourth aspect allows the second flowbetween the outlet of the cathode chamber and the exhaust port whenplaced in a first position, and allows the second flow between theoutlet and the fluid plenum when placed in a second position.

[0025] The fourth valve of the fourth aspect allows the third flow whenplaced in a first position and allows a fourth flow which controls aflow of fuel from a fuel supply cartridge to the anode chamber whenplaced in a second position.

[0026] In yet another aspect of the present invention, a fuel cellsystem includes an anode chamber having a fuel and a cathode chamber influid communication with an oxidizer. The cathode chamber includes aninlet positioned in a first end of the cathode chamber and an outletpositioned adjacent a second end of the cathode chamber. The fuel cellaccording to the fifth aspect further includes a proton conductingmembrane electrolyte separating the chambers and having an effluentgas-permeable portion allowing effluent gas produced in said anodechamber to flow into the cathode chamber, and a nozzle having an inletpositioned adjacent the gas-permeable portion in the cathode chamber andan outlet positioned adjacent outlet of the cathode chamber.

[0027] In yet another aspect of the present invention, a method forremoving water in a cathode chamber of a fuel cell, the fuel cellincluding an anode, a cathode chamber having an inlet and an outlet, anda membrane electrolyte having a gas-permeable portion, the methodincludes directing an effluent gas produced in the anode chamber fromthe gas-permeable portion into the cathode outlet at a pressure,establishing a low pressure region adjacent the outlet, and inducing aflow from the inlet through the cathode chamber and exiting the outlet.

[0028] The above aspect may further include equalizing the pressure toan ambient pressure adjacent the outlet of the cathode chamber.

[0029] The flows recited in the above aspects may be communicatedthrough the various elements via conduits and/or channels.

[0030] In addition, above aspects may include a controller for actuatingthe valves for controlling the flows.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0031] For a better understanding of the invention, reference is made tothe drawings which are incorporated herein by reference and in which:

[0032]FIG. 1 is a schematic diagram of a water management systemaccording to a first embodiment of the present invention.

[0033]FIGS. 2A-2B are schematic diagrams of modes of gas flow into adirect oxidation fuel cell system according to the first embodiment forthe present invention.

[0034]FIG. 3 is a schematic diagram of a water management systemaccording to a second embodiment of the present the invention.

[0035]FIGS. 4A-4B are schematic diagrams of gas flow and water returninto a direct oxidation fuel cell system according to the secondembodiment of the present invention.

[0036]FIG. 4C is a schematic diagram of a controller system for thewater management system for the embodiments of the present invention.

[0037]FIG. 5 is a schematic diagram of a water management systemaccording to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The term “low humidity gas” as used herein refers to ambient airor other gas containing substantially less than its saturation level ofwater vapor, that is, having a relative humidity of less than 100%.

[0039] As shown in FIG. 1, a direct oxidation fuel cell system 20includes a membrane electrolyte assembly 21 having a proton-conducting,electronically non-conductive membrane electrolyte 26 disposed betweenan anode chamber 22 and a cathode chamber 24. The exact shape of theanode chamber and cathode chamber may be defined by a “flow fieldchannel” which may be integrated into a flow field plate (not shown),which aids in distributing the fuel and the oxidizing agent to themembrane electrolyte. In this diagram, each surface of the membraneelectrolyte 26 is coated with electrocatalysts which serve as anodereactive sites 23 on the anode chamber side of the membrane and cathodereactive sites 25 on the cathode chamber side of the membrane. The anodeand cathode reactive sites facilitate the electrochemical reactions ofthe DMFC.

[0040] It is worth noting that the electrocatalysts may be provided inother areas within the anode and cathode chambers, and thus, theinvention is not limited to filel cells where the catalysts are providedon the membrane electrolyte.

[0041] Diffusion layers 27 and 28, may be included and positioned oneither side of the membrane. These layers provide a uniform effectivesupply of methanol solution (diffusion layer 27) to the anode reactivesites and a uniform effective supply of oxidizing agent (diffusion layer28) to the cathode reactive sites. Diffusion layers 27 and 28 on each ofthe anode and cathode sides of the membrane electrolyte also assist inproviding optimal humidification of the membrane electrolyte byassisting in the distribution and removal of water to and from themembrane electrolyte at rates that maintain a proper water balance inthe DMFC power system. Moreover, each layer may be used with a flowfield (not shown), to further aid in distributing fuel and oxidizer tothe respective reactive sites.

[0042] The form of the anode chamber may be defined by a flow fieldplate (not shown) which guides the fuel mixture over the anode diffusionlayer and also functions as a conductor (i.e., acts as the electricalanode), and an exhaust vent 30 which allows carbon dioxide createdduring oxidation of the fuel to pass out of the anode chamber.Similarly, the cathode chamber may include a flow field plate (notshown) which guides oxidizing agent in the chamber and also functions asa conductor (i.e., acts as the electrical cathode), an inlet 33 and anexhaust outlet 34 which allows air to flow through the cathode chamberso that an adequate supply of oxygen is insured for the reaction. Oneskilled in the art will appreciate that air may flow from inlet 33 tooutlet 34 and in the opposite direction, when the system is exposed toan ambient air pressure.

[0043] In a DMFC power system, an aqueous methanol solution, preferablya solution greater than 0% to about 100% methanol by volume, morepreferably between greater than 0% to about 30% methanol by volume andmost preferably approximately 3% methanol by volume, is used as thecarbonaceous fuel reactant. The methanol solution circulates past theanode reactive sites 23. Upon the application of an electrical loadbetween the flow field plates of the anode and the cathode chambers, themethanol solution disassociates, producing hydrogen protons andelectrons, and generating carbon dioxide as a first by-product of fueloxidation. Hydrogen protons migrate through the membrane electrolyte tothe cathode chamber while electrons pass through the external load. Theprotons and electrons then combine with oxygen in the cathode chamber toform water, the second by-product of the reaction. The electrons areretrieved by the flow field plate of the anode chamber and carriedthrough an external electrical load 29 to the flow field plate of thecathode chamber.

[0044] First Embodiment

[0045] In a first embodiment of the present invention, the flow ofcarbon dioxide is controlled through selective positioning of a ventvalve 32 and an air inlet valve 36. The vent valve 32 is a two-way valveincorporated in an exhaust vent conduit 30 for controlling venting andaccumulation of carbon dioxide (in conjunction with the air inlet valve)into a gas plenum 34. The air inlet valve 36 is a three-way valveincorporated at the intersection of an air inlet conduit 33 a and a gasplenum conduit 34 a, for controlling the flow of air and carbon dioxide(in conjunction with the vent valve 32) into the cathode chamber.

[0046] The positioning of the vent valve and the air inlet valvedetermine whether the water management system operates in an air inletmode or a flush mode. The air inlet mode allows air from the air inletconduit 33 a to flow into the cathode chamber and out of the exhaustoutlet 34 carrying water away and refreshing the available oxygen forreaction at cathode reactive sites.

[0047] During the flush mode, a significant pressure drop caused by thebuildup of carbon dioxide in the fluid plenum produces a high flowvelocity of carbon dioxide from the gas plenum into the cathode chamber24. This pressure drop also reduces the relative humidity of the carbondioxide stored in the plenum, so that it can more readily absorb waterin the cathode chamber. Thus, water is flushed from the cathode chamberby being blown out of the chamber by the pressure, and is evaporated dueto the lowered relative humidity of the carbon dioxide.

[0048]FIG. 2A illustrates the positions of vent valve 32 and the airinlet valve 36 during an air inlet mode. As shown, the vent valve 32 isopen between the anode chamber 22 and the gas plenum 34 to allow carbondioxide to accumulate in the gas plenum 34. The air inlet valve 36 isclosed to the gas plenum 34 and open between an air inlet and thecathode chamber 24, so that the plenum can operate as a storage tank forthe carbon dioxide and so that air may flow into the cathode chamber 24as required for fuel oxidation.

[0049]FIG. 2B illustrates the positions of the vent valve 32 and the airinlet valve 36 during a flush mode. In the flush mode, the vent valve 32is closed between the anode chamber 22 and the gas plenum 34 and the airinlet valve 36 is open to the gas plenum 34 and closed between the airinlet and the cathode chamber. This positioning allows the stored carbondioxide to flow out of the gas plenum and into the cathode chamber viaconduit 40.

[0050] The vent valve 32 is preferably actuated to the flush modeposition first, or concurrently with the air inlet valve 36. If the airinlet valve 36 is actuated before the vent valve, fuel may be expelledfrom the anode chamber into the cathode chamber adversely affecting theefficiency of the system as fuel is not used to generate power, but iswasted.

[0051] Because the membrane electrolyte operates more effectively withincertain humidification parameters, the flush mode will not dehydrate orremove substantially all water from the membrane electrolyte.

[0052] In order for the flush mode to operate effectively, apredetermined sufficient amount of carbon dioxide is necessary to flushthe water from the cathode reactive sites. Accordingly, the amount ofcarbon dioxide which has been generated must be determined.

[0053] In the present invention, the volume of carbon dioxide may bedetermined in the following ways:

[0054] (1) the level of fuel solution;

[0055] (2) the pressure level of the anode chamber;

[0056] (3) a time interval;

[0057] (4) power produced; and

[0058] (5) fuel concentration.

[0059] These methods and corresponding systems relate generally toactive control of the flow of carbon dioxide to the cathode chamber.Such active control is generally managed by a controller (digital oranalog) which actuates the valves for the various modes. However, it isworth noting that flow may also be controlled passively via reliefvalves, gas-permeable membranes, and other components that are wellknown to those skilled in the art.

[0060] Fuel Solution Level

[0061] As carbon dioxide is created and accumulates in the anode chamberand gas plenum, it pushes against the surface of the fuel solution. Apredetermined displacement of carbonaceous fuel correlates to apredetermined volume of carbon dioxide sufficient to cause or assist inthe removal of water from the cathode chamber. The system, however, isconfigured so that the predetermined displacement which determines whena flush mode is required still allows normal power generation. Thus,electricity production will not diminish as the predetermined level isreached.

[0062] Accordingly, when the predetermined displacement level isreached, a sensor sends a signal to a controller for actuating thevalves to the flush mode positions.

[0063] The valves may be reset to air inlet mode by either the sensor,which indicates that the fuel is no longer displaced the predeterminedamount, or by other means, including, but not limited to the use of atimer. The process is then repeated to continue generation ofelectricity.

[0064] Anode Chamber Pressure

[0065] A sufficient amount of carbon dioxide may be determined bydetecting the level of pressure in at least one of the anode chamber 22and the gas plenum 34. With this method and system, the pressure withinthe anode chamber is not fixed, rather it increases with the anodicoxidation of the fuel solution due to the generation of carbon dioxidewithin the anode chamber 24.

[0066] In the air inlet mode, the pressure of the anode chamber 22typically varies in relation to the water generated in the cathodechamber 24. The amount of carbon dioxide in the closed volume of theanode chamber, is directly related to the amount of water generated inthe cathode chamber. A predetermined level of pressure is associatedwith an amount of carbon dioxide suficient to remove said water from thecathode chamber.

[0067] When a pressure sensor (e.g., diaphragm type, or resistivebridge) within a wall of the anode chamber detects the predeterminedpressure level, a controller actuates the valves to the flush modepositions. It is worth noting that this method and system may notrequire a controller. Specifically, the valves may bepressure-responsive release valves, actuated in response to apredetermined pressure level, or other fuel cell system operatingcharacteristics.

[0068] Time Periods

[0069] Alternatively, the valves may be actuated between an air inletmode and a flush mode positions after a predetermined period of time haselapsed during fuel cell operation. The controller tracks the amount oftime when the cell is used for power. Since the power provided would beat a predetermined voltage/current, the amount of carbon dioxideproduced per unit time can be determined. Thus, after a predeterminedoperation time period has elapsed, the controller will actuate thevalves to flush mode positions.

[0070] Power Production

[0071] In a similar method and system, the carbon dioxide level may bedetermined by tracking how much electric energy has been produced by thecell. Accordingly, a predetermined amount of energy (power produced overa time interval) correlates with a certain amount of carbon dioxidegenerated. The controller tracks the amount of energy output andactuates the valves when the predetermined amount of energy has beenproduced.

[0072] Fuel Concentration

[0073] The fuel in the anode chamber is a mixture of carbonaceous fuel(i.e., methanol) and water. Unless otherwise compensated, as theoxidation process occurs, the fuel becomes less concentrated in thesolution, i.e., less fuel, more water. Because the concentration of thecarbonaceous fuel in the aqueous fuel solution and the amount of carbondioxide generated are inversely related (provided that adjustments forintroducing additional fuel are made), it is possible to determine howmuch carbon dioxide has been generated at the anode by measuring theconcentration of the fuel in aqueous solution. Thus, a fuelconcentration sensor (or sensors) sends fuel concentration signals tothe controller. When the concentration reaches a predetermined minimumindicating that fuel has been consumed to generate a sufficient amountof carbon dioxide to remove water from the cathode chamber, thecontroller actuates the valves to the flush mode positions.

[0074] Second Embodiment

[0075]FIGS. 3 and 4A-4B illustrate a second embodiment according to thepresent invention. In this embodiment, a recirculation system providesactive control for recirculating at least a portion of the watergenerated or transported across the membrane during fuel cell operation.By returning water to the anode chamber, fuel concentration is kept atan optimum level for efficient fuel cell operation, decreasing thevolume of water that must be stored with the methanol in a fuel supply,thus allowing the DMFC power system to have an increased energy density.The recirculation system is preferably used in conjunction with the gasflow control methods and systems described in the previous embodiment.

[0076] The recirculation system includes a drain conduit 50 connected tothe outlet of the cathode chamber, a drain valve 54, a drain outlet 51,a fluid plenum 57, a first fluid plenum conduit 52, a second plenumconduit 53, a return valve 56, a fuel supply conduit 59 and an anodesupply conduit 60 connected to an inlet of the anode chamber.

[0077] The drain valve 54 is a three-way valve positioned at theintersection between the drain conduit 50, the drain outlet 51 and thefirst fluid plenum conduit 52, and is used either to exhaust water andgaseous effluent from the cathode chamber to the environment (connectingdrain line 50 to the drain outlet 51), or to recirculate water removedfrom the cathode chamber to the cathode chamber (connecting drain line50 to the first fluid plenum conduit 52).

[0078] The return valve 56 is also a three way valve positioned at theintersection of the second fluid plenum conduit 53, the anode inletconduit 60 and the fuel supply conduit 59, and is used to keep aspecific amount of fuel solution in the anode chamber (connecting thefuel supply conduit 59 to the anode supply conduit 60), and torecirculate the water recovered from the cathode chamber into the anodechamber (connecting second fluid plenum conduit 53 with the anode supplyconduit 60). The valve 56 may also be used as a mixing chamber, formixing fuel solution with recirculated water from the cathode chamberfor supply to the anode chamber.

[0079] Depending upon the state of the system generally, and thedistribution of water in the DMFC power system, valves 54 and 56 may beactuated sequentially or simultaneously. Preferably, valves 54 and 56are used in conjunction with the previous embodiment, the drain valve 54and the return valve 56 include corresponding positions for the airinlet mode and the flush mode, and thus may be actuated upon detectionof the same process variables and/or physical conditions for vent valve32 and air inlet valve 36. Alternatively, the drain valve 54 and thereturn valve 56 may be activated independently of carbon dioxide flowcontrol process.

[0080]FIGS. 4A and 4B illustrate the positions of the drain valve 54 andreturn valve 56 during an air inlet mode (FIG. 4A) and a flush mode(FIG. 4B). As shown in FIG. 4A, during an air inlet mode, the drainvalve 54 exhausts water and carbon dioxide to the environment and thereturn valve 56 is closed to the fluid plenum 57 and open between thefuel supply cartridge 39 and the anode chamber 22. This establishes athroughput between the fuel supply 39 and the anode chamber 22 todischarge pressurized fuel into the anode chamber 22. Alternatively,during the air inlet mode, the return valve 56 (as shown in phantom) mayperiodically close to the fuel supply 39 and open to the fluid plenum 57establishing a throughput between the fluid plenum 57 and the anodechamber 22 to return water to the anode chamber 22 for adjustment of theconcentration of methanol solution. The return valve may also be closedto the fuel supply to halt the admission of new fuel during periods oflow or no power generation.

[0081] Upon detection of the sufficient volume of carbon dioxide for theflush mode (FIG. 4B), the drain valve 54 is actuated to open to thefluid plenum 57 establishing a throughput between the cathode chamber 24and the fluid plenum 57. Because pressure of the anode chamber 22 ishigher than ambient pressure of the cathode chamber 24 during the airinlet mode, upon actuation to the flush mode of gas flow control, thepressure of the anode chamber 22 drops substantially and equilibrateswith the pressure of the cathode chamber 24. Water flushed from thecathode chamber 24 in the flush mode, therefore, is propelled by theflow of carbon dioxide, assisting in the collection of water in thefluid plenum 57.

[0082] Generally, since only a portion of water may be required forrecirculation to the anode chamber, the drain valve 54 may remain opento the fluid plenum 57 for comparatively short intervals during theflush mode. Since the cathode and anode chambers, though connectedtogether through valves 32 and 36, are together closed to the ambient byvalves 36 and 54, some water pressure is maintained during recirculationof water from the cathode chamber 54 to the fluid plenum 57.Accordingly, if the drain valve 54 is closed to the plenum 57 (i.e.,allowing fluid communication between drain conduit 50 and the drainoutlet 51), the water within the fluid plenum can be discharged into theanode chamber upon opening of the return valve 56 between the secondfluid plenum conduit and the anode supply conduit. It should beunderstood that additional valve(s) or plenum(s) (not shown) betweenreturn valve 56 and anode chamber 22 may be used to control the flow ofrecirculated water and or fuel into anode chamber.

[0083]FIG. 4C illustrates a generic controller for actuating the valvesin both the first and second embodiments (independently and inconjunction). Accordingly, a controller 80, receiving signals fromsensor 90 (i.e., carbon dioxide levels) sends signals to valve actuators82, 84, 86 and 88 at the appropriate times for the air inlet mode, theflush mode, fuel supply and water recirculation.

[0084] Third Embodiment

[0085]FIG. 5 illustrates a third embodiment according to the presentinvention. In this embodiment, a passive control system using a lowhumidity gas produced in the anode chamber removes water from thereactive sites in the cathode chamber using an enhanced air flow system.

[0086] The system according to this embodiment includes a membraneelectrolyte assembly 62 disposed between an anode chamber 70 and acathode chamber 72 of a fuel cell system 60. The assembly 62 includes aproton-conducting, electronically non-conductive membrane electrolyte 64having a gas-permeable sector 66 selectively permeable to a desiredeffluent gas, such as carbon dioxide, but not to water or fuel. A gasejector 68 is connected to the cathode side of the gas-permeable sector66.

[0087] The gas ejector 68 having a first end 67 and a second end 69, maybe constructed preferably in a conical, parabolic or exponential shape.Each of these shapes causes the acceleration of the flow velocity ofcarbon dioxide as it travels from the first end to the second end. Toattain such a flow profile, the broadest portion of each shape ispositioned on the first end, with the narrowest portion positioned atthe second end.

[0088] The gas ejector 68 further includes a collar 65 disposed at thesecond end 69 of the gas ejector 68, and is preferably positioned toencompass a low pressure region at the exit of the gas ejector, createdby the flow of gas through the ejector. As air flows toward the lowpressure region, the air flow is entrained in the low pressure region.The collar 65 then carries the entrained air, together with any waterflushed from the cathode chamber and other effluents, toward an outlet79.

[0089] Accordingly, the system operates in the following manner. Air,supplied to the cathode chamber 72 from an external source, is deliveredby an air inlet 74 to the cathode chamber. The low pressure regionlocated at the second end 69 of the gas ejector draws air from the airinlet 74 toward the low pressure region. Air thereby is forced to flowinto and through the cathode chamber 72 at an enhanced velocity suchthat excess water accumulating in the cathode chamber, especially at thecathode reactive sites, is flushed or evaporated out of the chamber.

[0090] Having thus presented the present invention in view of the abovedescribed embodiments, various alterations, modifications andimprovements will readily occur to those skilled in the art. Suchalterations, modifications and improvements are intended to be withinthe scope and spirit of the invention. Accordingly, the foregoingdescription is by way of example only and is not intended as limiting.The invention's limit is defined only in the following claims and theequivalents thereto.

1-46. Canceled
 47. A fuel cell system comprising a fuel source, an anode chamber, a cathode chamber in gaseous communication with the anode chamber, and a protonically conducting membrane electrolyte.
 48. The fuel cell system according to claim 47, wherein no liquid is communicated between the anode chamber and the cathode chamber.
 49. The fuel cell system according to claim 47, further comprising at least one conduit for gaseous communication between the anode chamber and the cathode chamber.
 50. The fuel cell according to claim 49, further comprising a valve provided with the conduit to regulate gaseous communication between the anode chamber and the cathode chamber.
 51. The fuel cell according to claim 47, wherein the fuel source is comprised of a concentrated methanol solution.
 52. The fuel cell according to claim 51, wherein the solution includes a methanol concentration of greater than 50%.
 53. The fuel cell according to claim 47, further comprising a fuel concentration sensor integrated into the anode chamber.
 54. A fuel cell system comprising an anode chamber having a fuel and a cathode chamber in gaseous communication with the anode chamber via a conduit, wherein no liquid communication occurs between the anode chamber and the cathode chamber.
 55. A method for encouraging water and air exchange in fuel cell system, comprising collecting an effluent gas produced in the anode chamber of the filet cell and exhausting the collected gas through the cathode chamber to the ambient environment.
 56. An apparatus for encouraging water and air exchange in a fuel cell system, comprising collecting means for collecting an effluent gas produced in the anode chamber of the fuel cell and exhausting means for exhausting the collected gas through the cathode chamber to the ambient environment.
 57. A method of inducing airflow in a cathode chamber of a fuel cell system comprising directing an effluent gas produced in an anode chamber of a fuel cell at a pressure out of the fuel cell via a nozzle provided in an outlet in a cathode chamber of the fuel cell, wherein gases in the cathode chamber surrounding the outlet are induced to flow out the outlet area by the flow of the effluent gas.
 58. An apparatus for inducing airflow in a cathode chamber of a fuel cell system comprising directing means for directing an effluent gas produced in an anode chamber of a fuel cell at a pressure out of the fuel cell via a nozzle provided in an outlet in a cathode chamber of the fuel cell, wherein gases in the cathode chamber surrounding the outlet are induced to flow out the outlet area by the flow of the effluent gas. 